It's sort of the difference between books and magazines. But then, what is Twitter? Maybe a note pad--the kind we used to keep by the phone before voice mail? Or possibly a dead tree newspaper.IM is more like parallel work sharing chat or Wholey's fish tossing.

I can see the next S.A.T. question . . . LiveJournal:Book::FaceBook: _________

And then the kids can complain that it requires special cultural knowledge, because books and newspapers are ancient history.